Thermodynamics

Question 1

What is the difference between enthalpy and enthalpy change?

Answer 1

Enthalpy is absolute whereas enthalpy change is the difference between the initial and final enthalpy

Question 2

With what is enthalpy change measured?

Answer 2

Thermometer

Question 3

What is enthalpy change?

Answer 3

Heat energy change measured at constant pressure

Question 4

What is standard enthalpy change?

Answer 4

Heat energy change measured at constant pressure under standard conditions

Question 5

What are the standard conditions?

Answer 5

298K
100kPa
all compounds and elements in their standard states.

Question 6

What is the enthalpy of atomisation?

Answer 6

The enthalpy change when 1 mole of gaseous atoms are formed from an element in its standard state

Question 7

What is the enthalpy of bond dissociation?

Answer 7

The enthalpy change when 1 mol of a covalent bond is broken to form 2 moles of gaseous atoms.

Question 8

What is the formula for enthalpy of atomisation, relating enthalpy of bond dissociation?

Answer 8

Enthalpy of atomisation = enthalpy of dissociation/2

Question 9

What is the enthalpy change of first ionisation?

Answer 9

The enthalpy change when 1 mole of gaseous +1 ions are formed from 1 mole of gaseous atoms

Question 10

Describe the energy change in enthalpy of first ionisation

Answer 10

Endothermic
Energy is used to overcome electrostatic attraction between the positive nucleus and negative electron

Question 11

What is enthalpy change of second ionisation?

Answer 11

The enthalpy change when 1 mole of gaseous +2 ions are formed from 1 mole of gaseous +1 ions

Question 12

What is electron affinity?

Answer 12

Likeness for electron affinity

Question 13

What is the enthalpy change of first electron affinity?

Answer 13

The enthalpy change when 1 mole of gaseous -1 ions are formed from 1 mole of gaseous atoms

Question 14

Describe the energy change in the first electron affinity

Answer 14

Exothermic
Energy is released from the electrostatic attraction between the nucleus and electron being added

Question 15

What is the enthalpy change of second electron affinity?

Answer 15

The enthalpy change when 1 mole of gaseous -2 ions are formed from 1 mole of gaseous -1 ions.

Question 16

Describe the energy change in the second electron affinity

Answer 16

Endothermic
Energy required to overcome the repulsion between the negatively charged ion and electron being added

Question 17

What is the enthalpy change of formation?

Answer 17

The enthalpy change when 1 mole of a substance is formed from its elements in their standard states under standard conditions

Question 18

What is the enthalpy of lattice formation?

Answer 18

The enthalpy change when 1 mole of solid ionic compound is formed from its gaseous ions

Question 19

Describe the energy change in enthalpy of lattice formation

Answer 19

Exothermic
Energy is released from electrostatic attraction between oppositely charged ions

Question 20

What is the enthalpy of lattice dissociation?

Answer 20

The enthalpy change when 1 mole of solid ionic compound is broken into its gaseous ions

Question 21

Describe the energy changes in lattice dissociation

Answer 21

Endothermic
Energy is needed to overcome the electrostatic attraction between oppositely charged ions

Question 22

How to distinguish between values of lattice enthalpy of formation and dissociation

Answer 22

Lattice formation has -∆H
Lattice dissociation has +∆H

Question 23

What does lattice enthalpy indicate?

Answer 23

The strength of an ionic bond

Question 24

Why can lattice enthalpies not be directly measured?

Answer 24

It is difficult to measure temp rise in a solid

Question 25

Why is the Born-Haber cycle used to calculate enthalpy change?

Answer 25

Impossible to carry out experiments in which gaseous ions are spread out at infinite distances Reactions take place in the gaseous state, gas particles move randomly, so difficult to bring particles close together to measure.

Question 26

What factors affect lattice enthalpies?

Answer 26

Ionic charge Ionic radius

Question 27

How does ionic radius affect lattice enthalpy?

Answer 27

Smaller ions pack closely in a lattice, so attract strongly. Large ions are further apart in their lattice, so forces of attraction are weaker.

Question 28

How does ionic charge affect lattice enthalpy?

Answer 28

The larger the ionic charge the stronger the attraction, so the greater the lattice enthalpy

Question 29

What is theoretical lattice enthalpy?

Answer 29

Lattice enthalpy values calculated using the perfect ionic model (theoretical refers to the lattice enthalpy that is expected)

Question 30

What is experimental lattice enthalpy?

Answer 30

The lattice enthalpy values from a Born-Haber cycle

Question 31

What is the perfect ionic model?

Answer 31

Assumes ions are point charges and there is only electrostatic attraction (ionic bonding) between ions with no covalent character

Question 32

What does it mean if theoretical and experimental lattice enthalpies are similar?

Answer 32

The compound has almost 100% ionic character (almost no covalent)

Question 33

What is polarisation?

Answer 33

When the cation attracts the electrons of the anion, thus distorting the density in the anions towards the cation (hence the anion is polarised)

Question 34

What is polarising power?

Answer 34

The ability of a cation to attract electrons from the anion towards itself

Question 35

What is polarisability?

Answer 35

The ease with which an anion can be polarised

Question 36

What are the properties of a cation with large polarising power?

Answer 36

Small with big charge

Question 37

What are the properties of an anion with large polarisability?

Answer 37

Large with big charge

Question 38

Why does covalent character lead to different theoretical and experimental values?

Answer 38

Covalent character leads to stronger bonds (?)